The present invention relates to lasers and nonlinear frequency conversion techniques and, particularly, to a technique to convert infrared radiation to visible radiation using intracavity sum frequency generation.
Solid state lasers are a class of lasers which contain a solid state gain element. The gain element generally consists of a host material, which can be either a crystalline or amorphous or glass-like material, and a dopant or impurity ion distributed within the host material. The dopant ion, which is typically a transition element or rare earth element, is the primary determinant of the wavelength or wavelengths over which the laser can emit radiation. Typically, solid state lasers operate in the infrared region, that is, between 700 nm and 3 .mu..
However, it is desirable for numerous applications to use a visible laser. Because of the convenience of the solid state laser gain medium compared to either gaseous or liquid gain media, techniques have evolved to convert the infrared fundamental radiation from the solid state gain medium to visible radiation. Nonlinear optical conversion is commonly used to produce visible radiation from solid state lasers operating in the near infrared (IR). Wavelength in the blue are of particular interest for applications such as display technology, optical data storage, and underwater applications. The most common nonlinear optical conversion technique is a process called second harmonic generation, or doubling, in which the laser wavelength is directed through a nonlinear optical material. The emerging laser beam is at one-half the wavelength of the initial laser beam.
An important parameter for a nonlinear crystal is the phase matching condition. Optimum conversion from the fundamental wavelength to the second harmonic wavelength will occur when the wave vector mismatch between the fundamental wave and the generated wave is zero. This condition is termed "phase matching". Phase matching may be achieved in an anisotropic crystal by a suitable choice of direction of propagation and polarization relative to the crystalline axes.
Two different types of phase matching can occur in crystals. Type I phase matching refers to the process where the two fundamental waves have the same polarization. Type II phase matching occurs when the fundamental waves have orthogonal polarizations. Phase matching is achieved as a result of the dispersion of the nonlinear crystalline host. Dispersion refers to the dependence of the refractive index of a given material on wavelength. Therefore, a phase matched crystal is one which is phase matched for a specific nonlinear operation. For example, for second harmonic generation of Nd:YAG laser wavelength from 1.064.mu. to 532 nm one requires that the refractive index at 1.064.mu. and the refractive index in the same material at 532 nm be such that the phase relationship between the fundamental wavelength and the generated second harmonic wavelength remain unchanged as the two waves propagate along the length of the crystal.
When phase matched second harmonic generation is achieved by propagating the fundamental wavelength along a direction different from a principal axis of a birefringent crystal it is termed "critical phase matching." When critical phased matched second harmonic generation is used with a focused beam, there is a phase mismatch of the wave vector for small deviations from the phase match direction due to the finite divergence of the beam. Because the efficiency of the nonlinear conversion process is a function of the power density within the nonlinear crystal, focusing is generally desirable to achieve high conversion efficiency.
When the phase matching angle is 90 degrees for a particular nonlinear process in a given material, it is termed "noncritical phase matching" (NCPM). In such a case, effects of beam divergence vanish. That is to say, a strongly focused beam in an NCPM crystal does not have the phase mismatch problems as is evident in critical phase matching. In addition, the walk-off angle which is the direction of energy flow of the fundamental and second harmonic beams, is zero, meaning that these two beams propagate collinearly within the crystal. It is obvious, then, that NCPM is the most favored and desirable means of operating a nonlinear material. One means by which NCPM can be obtained is by adjusting the temperature of the nonlinear crystal to the point where the refractive index of the fundamental wavelength equals that of the second harmonic wavelength.
Second harmonic generation is a special case of a more general nonlinear optical conversion process known as sum frequency generation. In second harmonic generation, two waves of the same wavelength are combined to produce a single wave of a wavelength one-half the original fundamental wavelength. In sum frequency generation two fundamental waves of different wavelengths are combined to produce a single wave. The wavelength produced by sum frequency generation is determined by the following equation: ##EQU1## where .lambda..sub.1 represents one of the fundamental wavelengths, .lambda..sub.2 represents the second of the fundamental wavelengths and .lambda..sub.3 represents the converted or summed wavelength. It can be seen, then, that second harmonic generation is a degenerate case of sum frequency generation where .lambda..sub.1 =.lambda..sub.2. The fundamental principles of nonlinear optics summarized briefly above are well known and are discussed in detail in the literature. See, for example, G. D. Boyd and D. A. Kleinman, Journal of Applied Physics, vol. 39, p. 3597, 1968.
Although doubling can be an efficient means for conversion to the blue, the nonlinear optical material KT.sub.i OPO.sub.4 (KTP) is non-critically phase matched at room temperature for sum frequency generation (SFG) at 808 nm and 1.064.mu., see, for example, K. Kato, IEEE J. Quantum Electronics, vol. QE-24, p. 3, 1988. The generated wavelength is 459 nm. There are several compelling advantages to recommend the SFG process over direct doubling to 459 nm. For one, the required fundamental wavelengths can be obtained efficiently. Nd:YAG operates at 1.064.mu. while several lasers, including Ti.sup.3+ :sapphire, AlGaAs laser diodes and Cr.sup.3+ -doped crystals produce efficient output at 808 nm. In addition, KTP is a robust, mature and efficient nonlinear crystal which is readily available in excellent quality from a number of commercial suppliers. And finally, KTP has an exceptionally wide angular and temperature bandwidth for non-critical phase matched sum frequency generation; see, for example, J. -C. Baumert, F. M. Schellenberg, W. Lenth, W. P. Risk and G. C. Bjorklund, Appl. Phys. Lett., vol. 51, p. 2192, 1987.
Typically, sum frequency generation requires two different laser sources. Because the efficiency of the sum frequency generation process depends on the power density within the optical crystal for CW operation, one requires extremely small focused spot sizes within the nonlinear sum frequency generating crystal. Using two different laser sources generally leads to problems involving the alignment of the beams to the high degree of accuracy required by the small spot sizes within the nonlinear crystal. In addition, when using two separate laser sources, there are inefficiencies that result from mismatching of the spatial modes of the two lasers in terms of size, shape and intensity distribution.
One technique for avoiding the use of two separate lasers for the sum frequency generation process in which 808 nm and 1.064.mu. are combined to produce 459 nm blue output is to use a diode pumped Nd:YAG laser which uses the residual (unabsorbed) 808 nm pump radiation for sum frequency generation. See, for example, W. P. Risk, J.-C. Baumert, G. C. Bjorklund, F. M. Schellenberg and W. Lenth, Appl. Phys. Lett., vol. 52, p. 85, 1988. In this type of sum frequency generation a laser diode at 808 nm is used to pump a Nd:YAG laser which operates at 1.064.mu.. The residual or unabsorbed 808 nm pump light is then circulated within the Nd:YAG laser resonator cavity which also includes a sum frequency generation KTP crystal. In such a system there is only one active laser, the laser diode. Since the Nd:YAG is optically excited by the laser diode and in essence serves as a frequency conversion device to convert some of the 808 nm light to 1.064.mu. light, in this manner one might conclude that sum frequency generation is achieved with the use of only one active laser. A patent by Baumert et al., U.S. Pat. No. 4,791,631, describes this concept in detail. A variation on this type of sum frequency generation process is to have an additional laser diode or laser diodes which are not used to pump the Nd:YAG directly but are used to introduce additional 808 nm light into the laser resonator which contains the Nd:YAG crystal and the KTP nonlinear crystal. In this case a separate laser diode is used to pump the Nd:YAG laser.
It should be noted that for CW sum frequency generation the use of the KTP crystal within a resonator is essentially a requirement which stems from the necessity of having very nigh power densities to achieve efficient generation of 459 nm light. Therefore, the KTP crystal used for cw sum frequency generation usually receives focused light at 808 nm and 1.064.mu. within a resonator cavity. Using the KTP crystal inside a cavity is desired because the light contained within a cavity, which is forced by highly reflective end elements to oscillate back and forth, will have a much higher power and therefore power density than light outside the cavity.
A second patent by Dixon et al. U.S. Pat. No. 4,879,723 describes another version of the concept patented earlier by Baumert et al. In the Dixon et al. patent a laser diode pumped Nd:YAG laser is established in a cavity containing the KTP crystal similar to the Baumert patent. In addition, the output of a second laser diode is introduced into this same cavity to provide a separate source of 808 nm fluence. In the Dixon et al. patent a possibility of high modulation rate of the 459 nm light is considered.
Sum frequency generation utilizing a laser diode has a unique set of difficulties, particularly for scaling to a higher power. These difficulties stem from the broad spectral bandwidth and poor beam quality that is typically associated with high power laser diodes. Laser diodes on the order of 1 Watt or more are typically multi-spectral devices which limit the efficiency of sum frequency generation at a specific blue-green wavelength. In addition, these high power laser diodes typically are multi-transverse mode devices, as they arise from gain-guided wide-stripe architectures. As a consequence of the large number of transverse modes, it is not possible to focus the output of the high powered 808 nm laser diode into a small enough spot to produce efficient sum frequency generation. In addition, the spatial mismatch between the focus spot of a typically astigmatic high transverse mode laser diode and a TEM.sub.00 output of a Nd:YAG laser further prohibits good efficiency when one is trying to obtain high power.
For CW sum frequency generation in which 808 nm and 1.064 nm are summed to produce 459 nm by using a KTP nonlinear crystal, an additional problem arises in the types of approaches represented by the Dixon et al. and Baumert et al. patents cited above. In those two patents, the cavity which is used to resonate the two fundamental wavelengths at 808 nm and 1.064.mu. to produce the high intracavity fluences required for efficient frequency generation also contains the Nd:YAG laser gain element. However, the Nd:YAG element absorbs strongly at 808 nm and therefore limits the intracavity fluence at that wavelength. Subsequently the overall conversion efficiency is reduced. Several variations on the general techniques proposed by the Dixon et al. and Baumert et al. patents have been published, all of which suffer from the problem of having a Nd:YAG absorbing gain element within the cavity with which one is trying to obtain high intracavity fluence at 808 nm. See, for example, D. W. Anthon and G. J. Dixon, M. G. Ressl, and T. J. Pier, Proceedings of the SPIE, vol. 898, p. 68, 1988; W. P. Risk and W. Lenth, Appl. Phys. Lett., vol. 54, p. 789, 1989; and P. N. Kean and G. J. Dixon, Optics Letters, vol. 17, p. 127, 1992.
Since good efficiency dictates the use of intracavity sum frequency generation (or "mixing") to take advantage of the high circulating power, absorption of the 808 nm fluence in the cavity by the Nd:YAG gain element contained within that cavity will counteract the enhancement of the 808 nm fluence within the cavity and reduce the overall conversion efficiency. One solution to the problem has been suggested in which optically thin Nd:YAG slabs are used which do not absorb as strongly in a single pass as typically much longer Nd:YAG rods do. See, for example, P. N. Kean and G. J. Dixon, Optics Letters, vol. 17, p. 127, 1992. The use of the optically thin slab mitigates the problem but does not eliminate the intracavity absorption of 808 nm fluence.
A different type of solution has been proposed in which sum frequency generation occurs in an external cavity. In this type of approach a laser diode pumped Nd:YAG laser is established. The output of this laser is introduced into a second resonator cavity which contains only a KTP crystal and highly reflective mirrors. The 1.064.mu. fluence from the diode pumped Nd:YAG laser circulates in this cavity. In addition, the output of the laser diode at 808 nm is also introduced into the resonator containing only the KTP crystal. Thus, both 1.064.mu. and 808 nm fundamental radiation can circulate with high power density and produce efficient 459 nm radiation. See, for example, W. P. Risk and W. J. Kozlovsky, Optics Letters, vol. 17, p. 707, 1992. However, it is to be noted that generation in an external cavity, although circumventing the absorption problem in which the Nd:YAG gain element absorbs the 808 nm circulating fluence, introduces the alignment and mode matching difficulties which were referred to above. This sort of technique for generating 459 nm light is therefore subject to losses in efficiency due to spatial overlap of two beams from two separate lasers.
Thus in accordance with this inventive concept, a continuing need has been found in the state of the art for a technique for intracavity sum frequency generation using 808 nm and 1.064.mu. to produce 459 nm output with a nonlinear crystal composed of KTP which is efficient, scalable to higher powers, insensitive to alignment problems, arises from a single laser source, and contains no elements within the cavity that reduce the intracavity fluence at 808 nm.